Fuel cells have been known as systems for converting chemical energy derived from raw fuel materials directly to electrical energy. A typical fuel cell consists mainly of a pair of opposing porous electrodes--a fuel electrode and an oxidant electrode--separated by an electrolyte layer holding an electrolyte, in which electricity is generated between the two electrodes by an electrochemical reaction triggered when a fuel, e.g. hydrogen, is directly fed to the back of the fuel electrode and an oxidant, e.g. air, is fed to the back of the oxidant electrode. In operation, amounts of electrical energy are produced at a high conversion rate while both the fuel and the oxidant are supplied continuously. Besides, because of their favorable characteristics to energy saving and environmental conservation, the fuel cells have been studied for practical use.
In such a fuel cell, the fuel (an anode active substance) for feeding to the fuel electrode is commonly hydrogen which is supplied in the form of a fuel gas, containing hydrogen as a primary component, converted by steam reforming reaction from a raw fuel material such as methane, ethane, propane, butane, natural gas, naphtha, kerosine, gas oil, liquefied petroleum gas (LPG), town gas, etc.
It is understood that a sulfur component in the raw fuel material poisons a steam reforming catalyst (e.g. Ru catalyst or Ni catalyst). If the raw fuel material contains, for example, 0.1 ppm of sulfur, about 90% of the surface of a Ru or Ni catalyst will be covered with sulfur in a short time thus losing catalytic activity. Hence, the raw fuel material is initially desulfurized prior to steam reforming process.
Accordingly, a conventional fuel cell power generation system comprises: a desulfurizing unit for desulfurizing a raw fuel material; a fuel reforming section for converting the desulfurized raw fuel material by steam reforming into a fuel gas containing a primary component of hydrogen which is in turn fed to the fuel electrode of a fuel cell; an oxidant feeding section for feeding an amount of oxygen (air) to the oxidant electrode of the fuel cell; and a fuel cell unit for generating electricity by an electrochemical reaction between hydrogen from the fuel gas and oxygen. The desulfurization of a raw fuel material prior to steam reforming process is commonly carried out by hydrogenating desulfurization in which organic sulfur in the raw fuel material is decomposed by hydrogenolysis, for example, at 350.degree. to 400.degree. C. in the presence of Ni-Mo or Co-Mo catalyst and then, resultant H.sub.2 S is removed by adsorption on ZnO at 350.degree. to 400.degree. C.
FIG. 1 is a system diagram showing an outline of substantial arrangement of a typical fuel cell power generation system (PC18) which contains a desulfurizing unit for performing the hydrogenating desulfurization and a steam reforming unit. As shown, a raw fuel material 1 is mixed with a fuel gas, which contains hydrogen as a primary component, introduced from a carbon monoxide shift converter 5, described later, and fed to a hydrogenating desulfurization reactor 2a. The hydrogenating desulfurization reactor 2a contains, from entrance, a hydrogenation layer filled with e.g. Ni-Mo or Co-Mo catalyst and an adsorption layer filled with an adsorption desulfurizing agent such as ZnO. The raw fuel material 1 mixed with a portion of the fuel gas derived from the carbon monoxide shift converter 5 is heated up to 350.degree. to 400.degree. C. by a heater (not shown) and then, hydrogenated at the hydrogenation layer so that a sulfur component of the raw fuel material is converted into H.sub.2 S, which is in turn adsorbed into the adsorption layer, for desulfurization of the same. The desulfurized raw fuel material 1 is mixed with steam in a mixer 3 and transferred to a steam reformer 4 where it is converted by steam reforming reaction into a fuel gas containing hydrogen as a primary component. The resultant fuel gas is then transferred to the carbon monoxide shift converter 5, filled with shift catalyst, where carbon monoxide is converted into hydrogen and carbon dioxide: this procedure is needed for preventing poisoning by carbon monoxide upon the catalyst (e.g. platinum catalyst) of a fuel electrode 7 and also, enhancing efficiency in the conversion to hydrogen. Most of the fuel gas from the carbon monoxide shift converter 5 is supplied as a fuel to the fuel electrode 7 in a phosphoric acid electrolyte fuel cell unit 6 while a small portion of the same is returned to the hydrogenating desulfurization reactor 2a. Hydrogen in the fuel gas introduced into the fuel electrode unit 6 then reacts in electrochemical process with oxygen in the air 9 which is supplied by a compressor 8 into an oxidant electrode 10. As a result of the reaction, a portion of the fuel gas is consumed while a by-product of water is created, and electrical energy is produced which is almost equivalent to a current provided to an electric load 19.
The fuel gas discharged from the fuel electrode 7 is transferred to a burner 11 in the steam reformer 4 where it is mixed with a portion of the air 9 supplied from the compressor 8 and burned for heating the steam reformer 4. A resultant exhaust gas containing steam is transferred from the burner 11 via a heat exchanger 12 to a condenser 13 where it is separated into water and gas. The separated gas is discharged. The condensed water is added to a water supply line 14 which is fed water via a water supply pump 15 and a cooling water pump 16 to the fuel cell unit 6 for cooling purpose. The cooling water is circulated from the fuel cell 6 via a heat exchanger 17 to a gas/water separator 18 for separation into steam and water. The separated water is then returned across the cooling water pump 16 to the fuel cell unit 6 for recirculation and the separated steam is transferred to the mixer 3, where it is mixed with the raw fuel material 1, and then, fed to the steam reformer 4 for use in the steam reforming reaction.
The aforementioned prior art fuel cell power generation system has however some disadvantages to be overcome. At the hydrogenating desulfurization step for desulfurizing the raw fuel material, if organic sulfur contained in the raw fuel material is excessive in amount or hard to be decomposed, e.g. thiophene, it will be slipped off without being decomposed and fail to be adsorbed by ZnO thus passing away. This phenomenon is also unavoidable when the raw fuel material is a gaseous fuel such as a town gas which contains an odorant of hardly decomposable, non-adsorbable organic sulfur such as dimethylsulfide. The hydrogenating desulfurization catalyst is tended to provide catalytic activity at a temperature of more than 350.degree. C. and will hardly react in response to a change in the fuel cell load. Also, the catalyst requires an extra heating device and a flow controller for activation with no warming-up period thus will rarely be reduced in the size.
During the adsorption desulfurization, chemical equilibrium is involved as expressed by: EQU ZnO+H.sub.2 S.revreaction.ZnS+H.sub.2 O EQU ZnO+COS.revreaction.ZnS+CO.sub.2.
Hence, the amount of H.sub.2 S and COS remains not less than a certain value. Particularly, this will be emphasized while H.sub.2 O and CO.sub.2 are present. Also, if the desulfurizing section in the entire system is unstable during startup and/or shutdown procedures, sulfur may escape from the adsorption desulfurization catalyst thus increasing the concentration of sulfur in the raw fuel material. For preventive purpose, the desulfurization of the prior art is executed in which the raw fuel material contains surfur in the level of several ppm to 0.1 ppm after refinement. Accordingly, poisoning of the steam reforming catalyst can not be fully suppressed and constant long-run operation of fuel cell will be hardly ensured.
In the prior art fuel cell power generation system, the fuel gas fed to the fuel electrode consists mainly of: hydrogen, a reaction product generated by steam reforming reaction; carbon dioxide generated by carbon monoxide shift reaction; and surplus steam which remains unused during the steam reforming reaction. When a partial pressure of hydrogen in the fuel gas is increased, the efficiency of power generation in the fuel cell becomes improved. It is however difficult to reduce the amount of carbon dioxide in the fuel gas which is released by the carbon monoxide shift reaction. Hence, the task of increasing the hydrogen partial pressure and improving the power generation efficiency in the fuel cell is embodied by decreasing S/C (mole numbers of steam per carbon mole in hydrocarbon in the raw fuel material) during the steam reforming reaction and also, the surplus amount of steam. However, when the S/C is reduced, the concentration of carbon monoxide, a product of steam reforming reaction, in the fuel gas increases even after shift reaction in the carbon monooxide shift converter. The carbon monoxide s now going to poison the catalyst of the fuel electrode in the fuel cell, particularly a platinum catalyst which is commonly employed in a phosphoric acid electrolyte fuel cell operable at a lower temperature, which is in turn deteriorated in properties. In other words, using a fuel gas containing a high concentration of carbon monoxide causes a decrease in the power generation efficiency of a fuel cell.
As described above, the steam reforming catalyst is poisoned by sulfur in the raw fuel material and its catalytic activity is declined, whereby deposition of carbon onto the catalyst surface will be stimulated. For prevention of this action, the S/C is increased in the prior art. If the S/C is decreased, the catalyst activity declines and carbon will be deposited on the catalyst surface causing increase of differential pressure and simultaneously, the raw fuel material itself will be fed to the fuel cell while being incompletely decomposed, preventing the fuel cell to perform a long-run operation without difficulties. Also, the catalyst installed in the steam reformer has to be increased in amount for compensation of a loss caused by the sulfur poisoning of catalyst. As the result, the steam reformer remains not decreased in the size and the overall size of the fuel cell can hardly be minimized.
As understood, although lower S/C in the fuel cell power generation intends to increases the hydrogen partial pressure in the fuel gas, it is difficult to reduce the S/C because the aforementioned drawbacks ar no more negligible. For example, the S/C cannot be less than 3.5 when Ni catalyst is applied as the steam reforming catalyst nor below 2.5 when Ru catalyst, which provides a higher catalytic activity, is applied. The S/C is commonly adjusted for steam reforming reaction to more than 3 with the Ru catalyst and 4 with the Ni catalyst. Thus, the amount of steam in the fuel gas will be increased preventing rise in the partial pressure of hydrogen.
The present invention is directed, for the purpose of elimination of the foregoing disadvantages attribute to the prior art, to an improved fuel cell power generation system in which a raw fuel material is desulfurized at a high enough degree to prevent the deterioration of steam reforming catalyst even if the S/C is low so that steady long-run operation can be ensured and a process of producing a fuel gas which is high in the partial pressure of hydrogen.